This invention relates to purifying used motor, hydraulic, gear, and other oils. More particularly, it relates to a new process for purifying spent oils employing a combination of chemical treatment and membrane process technology. Because of the very low level of contaminant, e.g., ash and metals, resulting from this new process, the oil recovered can be re-used in its original applications or used for fuel applications.
Nationwide about 1.3 billion gallons of used motor and other oils are generated annually. About 70% is currently being recycled as fuel. The remainder is not collected or is lost in usage. These oils can contain up to 15 to 20% of impurities, such as water, sludge, carbonaceous particles and oxidation products. When reused as fuel, these contaminants create air pollution problems. Generally, the used motor oil contains 0.5 wt. % of ash residue after combustion.
By oil as used herein is meant to include all kinds of synthetic and mineral oils including crude oil, and particularly spent or used engine oils, hydraulic oils, cooking oils and the like from which fractions of contaminants such as debris, metal and ash can be removed to permit its reuse as fuel or for other applications.
Most often, the used oil is an uneven product of oil collected from several sources. The main contaminants typically are:
Water from engines and storage. PA1 Dissolved gasoline and gas-oil, resulting from use in engines. PA1 Solvents, aromatics and cleaning fluids. PA1 Sediments, consisting of: carbonaceous particles, resulting from poor combustion of motor fuels; metal particles, brought in by wear and corrosion, external dust. PA1 Lead from gasoline and anti-knock additives. PA1 Polymeric additives, for viscosity improvement or sludge dispersion (polymethacrylates, polysuccinmides). PA1 Non polymeric anti-oxidants, anti-wear or detergent dispersing additives (zinc dialkyl-dithiophosphates, calcium or barium salicylates, acoylphenates or sulfonates).
There are several processes available for purifying used oil. The ash and other contaminants can be removed by chemical cracking, membrane-based filtration and evaporation.
The membrane-based process for waste oil purification has been explored and disclosed to reduce ash content. For example, in Canadian Patent 1,168,590, used motor oil was treated with Al.sub.2 O.sub.3 and MgO mixed oxide ultrafiltration membranes with pore sizes ranging from 50 to 250 .ANG. to remove impurities and additives. Ash content of the product is listed as &lt;0.005 wt. % from the feed containing 0.35 wt. %. However, substantial heavy metals remain in the product, such as from 8 to 241 ppm of lead, and 6 to 152 ppm of iron. U.S. Pat. No. 3,919,075 discloses using polymeric ultrafiltration membranes, and lead content was reduced from 460 to 260 ppm, and the ash content was reduced from 20 to .about.0.07 wt. % (Example 14 and 15). The membranes reported in the literature show the capability of significant reduction in the ash content; however, substantial heavy metals such as lead still remain. Evidently some ash precursors and/or heavy metal contaminants are dissolved or well dispersed in waste oil. Therefore, even inorganic membranes with the pore size as small as 50 .ANG. were unable to achieve significant removal of selected heavy metals. Often throughput is sacrificed because of the use of the membrane with such a small pore size to treat viscous waste oil. An increase in operating temperature can alleviate the throughput limitation; however, it introduces another disadvantage: generation of odorous compounds, such as mercaptans. In short, there is no practical solution available to achieve a substantial ash and heavy metal removal with the existing technology. Thus, there is a great need to provide an improved process which will remove ash and metals to a very low level and permit reuse of the oil in the original applications. The present invention provides such a process.